The present invention relates generally to flame detectors and detection methods based on electric current rectification properties of a flame, and, more specifically, to such a detector and detection method employing periodic pulses of alternating current excitation in a manner to permit detection of inoperable electrical components which could result in flame detection errors.
Combustion systems, such as those used in furnaces, boilers and other appliances in which controlled burning takes place, commonly utilize automatic control systems to ignite, control the flow of, and monitor the combustion of a fuel, such as natural gas, oil, etc. One of the required functions for such an automatic control system is to monitor the flame, and, in its absence, terminate the flow of the fuel to avoid serious hazards. Thus, it is essential that any flame detection apparatus in such a system be designed and constructed to operate in a highly reliable manner.
One well known technique for detecting the presence of a flame is based on electrical properties associated with the flame. As a flame burns, it produces an ionized region in its vicinity, thereby providing an electrically conductive medium. This property can be utilized in conjunction with a probe placed into the flame, and a grounded metal burner to produce a usable electrical signal. If such apparatus is constructed with an effective grounded burner area greater than the effective probe area, typically in at least a 4 to 1 ratio, the flame will exhibit electrical characteristics somewhat similar to those of a diode in series with a 10 megohm resistor. If an alternating current signal is injected into the flame by the probe, the signal will be rectified by the flame. Appropriate filtering and amplification circuitry may then be employed to extract the rectified signal.
Two practical applications of this technique are illustrated in FIGS. 1 and 3, which are schematic diagrams of previously known systems. FIG. 1 illustrates a shunt topology implementation in which an alternating current drive generator is connected electrically in parallel with the burner/flame/probe system. If a flame is present, a voltage divider will be formed during the positive excursions of the AC drive signal. This will have the effect of developing a negative bias on the AC signal at the input of a flame filter/amplifier generally designed to remove the AC component, leaving only a DC signal indicative of the presence or absence of a flame. A signal indicating absence of a flame is typically used to terminate the supply of fuel to the burner. It may also be used to produce a visual warning of system failure.
FIG. 3 illustrates a series topology implementation of the prior flame rectification detection technique. The requirements for the detector system components are generally the same as for those in the shunt topology implementation, except that current, rather than voltage, signals are processed.
Because the circuitry for performing the filtering and amplifying is considered safety related, industry standards require that first and second level failure modes must not compromise its function. This requirement has led to the use of redundant circuit elements, adding expense, leakage paths and crowded circuit board conditions. Thus, a need exists for a flame detector design which provides fail safe operation without requiring redundant components.